This seems about right. My install of SitS is close to 3 GB in size.
Now, i know it's odd to see an installer, that basically has a compression ratio of close to 90%, but judging from the game files, this is not that crazy actually.
If you look at the game files, they are mostly made up of hex files that contain the game code. Files like this (hex or ascii, text if you will) work really well when compressing (you can test this with any text file on your system if you will).
AAA games contain lots of other file types, that are mostly lacking in SitS, which are precompressed, and usually can't be compressed much further. Textures in a 3d game, or sound files already come in a compressed format (think mp3 for sound, for textures it's .dds). put these in a compressed archive and you game little size optimization.
But SitS contains very little of those, but instead a lot of complex level data with branching dialog trees and tons of "If-Then" conditions and what not.
Tl,dr: because SitS is a pixel art game with complex gameplay mechanics, it's one of the few games where a 90% installer compression ratio is not unexpected.
makes sense. clears it up for me too. it is a very old game. most security "monitor" software have little to no data on such old titles. the graphics are not what it is mostly made of, but the gameplay and data.